Toxicology Brief: Helping animals exposed to the herbicide paraquat - Veterinary Medicine
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Toxicology Brief: Helping animals exposed to the herbicide paraquat


VETERINARY MEDICINE


Because systemically absorbed paraquat is eliminated primarily through renal excretion and the presence of oliguric renal failure markedly contributes to paraquat accumulation in the lungs, maintaining urine production is critical when treating an animal with paraquat toxicosis.2 Forced diuresis can remove large quantities of circulating paraquat if it is initiated within a day or so of ingestion.2,27-30 However, forced diuresis carries with it the risks of electrolyte disturbances and exacerbation of paraquat-induced pulmonary edema. Particular caution is required during the first 24 hours after ingestion, especially if oliguria is present.2

Antioxidant therapy has been extensively studied in experimental models and cases involving people with paraquat toxicosis with variable results.31 Recent studies using trimetazidine (an anti-ischemic), S-carboxymethylcysteine (a respiratory drug), propofol, and epigallocatechin gallate (from green tea) have shown promising results, but clinical experience with these agents is limited, and little controlled clinical trial data are available.31-34 Because of the risk of enhanced oxidative effects, oxygen administration in patients with paraquat toxicosis should be avoided except when necessary for comfort (e.g. patients in respiratory distress).2 Hypoxic ventilation has been used in paraquat poisoning cases in people, but its effectiveness has not been extensively studied.35,36 Collagen synthesis inhibitors may offer some control or prevention of pulmonary fibrosis, but their use has not been extensively studied in field conditions.37 Corticosteroids, immunosuppressants, vitamins, ?-blockers, alkylating agents, chlorpromazine hydrochloride, ?-tocopherol, superoxide dismutase, glutathione peroxidase, and nitric oxide inhalation have all been used to treat paraquat toxicosis with little clearly documented effectiveness.2,14,38,39 Currently available immuno-antidotes are ineffective.40

Prognosis and prevention

Despite treatment, the overall prognosis for paraquat toxicosis is poor. Disappointingly, survival from paraquat poisoning may be more strongly associated with the circumstances of the poisoning rather than any treatment administered.35 Situational factors associated with higher survival rates in people include inhalation or dermal exposure, ingestion of less than 35 mg/kg, a young age at the time of poisoning, the time between paraquat ingestion and the last meal (because paraquat is adsorbed and neutralized by foodstuffs), accidental ingestion rather than homicidal intention, ingestion of diluted materials rather than liquid concentrates or granular formulations, and aggressive treatment within two to five hours of ingestion.2,15,28,35 In people, a lack of caustic gastric lesions; lower urine and plasma paraquat concentrations; lesser degrees of leukocytosis, acidosis, and respiratory distress; and absence of renal, hepatic, and pancreatic failure at the time of admission are all considered to be good predictors of survival.15,28,35

Because of the severe consequences of paraquat toxicosis, early diagnosis and aggressive treatment are paramount if the survival prospects of a severely poisoned patient are to be improved. Given the limited effectiveness of current treatment modalities, the best solution to the problem of paraquat poisoning in companion animals is to prevent exposure.

REFERENCES

1. Cope, R.B. et al.: Seven cases of fatal paraquat poisoning in Portland Oregon dogs. Vet. Human Toxicol. (in press).

2. Akintonwa, D.A. et al.: Paraquat and diquat. Environ. Health Criteria 38:1-131; 1984.

3. Bischoff, K. et al.: Malicious paraquat poisoning in Oklahoma dogs. Vet. Hum. Toxicol. 40:151-153; 1998.

4. Longstaffe, J.A. et al.: Paraquat poisoning in dogs and cats—Differences between accidental and malicious poisoning. J. Small Anim. Pract. 22:153-156; 1981.

5. Cruickshank, A.K.: Metaldehyde and paraquat poisoning. Vet. Rec. 111:149-150; 1982.

6. Johnson, R.P.; Huxtable, C.R.: Paraquat poisoning in a dog and cat. Vet. Rec. 98:189-191; 1976.

7. Kelly, D.F. et al.: Pathology of acute respiratory distress in the dog associated with paraquat poisoning. J. Comp. Pathol. 88:275-294; 1978.


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Source: VETERINARY MEDICINE,
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